Make frame state available before rendering frame

Without this change, tiles that are enqueued on the first rendering will always get priority `ol.TileQueue.DROP` because `map.frameState_` is not yet defined (meaning a single render call doesn't result in any tiles loading).
Only track tiles that are actually wanted

Instead of keeping track of wanted tile ranges, we can instead track wanted tiles individually.  This provides enough for the map to know how to prioritize tiles and should be more efficient (no extra calls to extend tile ranges or check for tile containment within a range).
Don't repeatedly render while waiting for tiles to load

Instead of going into an unnecessary animation loop, we can simply wait to be notified when tiles load.

The WebGL renderer still sets frameState.animate true, but I think this too should be unnecessary (full page example works without it, side-by-side example shows it cannot yet be removed).
Only listen for first event
Merge pull request #184 from tschaub/less-animation

Avoid unnecessary animation loop while tiles load